A readily manufactured device which will dependably release an active material (e.g., a pharmaceutical agent, a cleanser or a deodorizer) at a zero-order rate into a fluid medium (gaseous or liquid) has remained an elusive goal, particularly when the device is in the form of a tablet for controlled in vivo release of a pharmaceutical agent into a biological fluid (e.g., the fluid of the gastrointestinal tract).
An early proposed method was that of Jacobs, U.S. Pat. No. 3,113,076 (1963) in which the drug was combined in a suitable carrier and tablets obtained by an extrusion method. The principle was to form tablets with approximately equal outer and "inner" surfaces, the latter accessed by aperture(s). As the exterior surface is dissolved, the area decreases, while as the inner surface dissolves, the surface area increases. Absent diffusion effects respecting the interior surface, the total surface, and thus rate of solution, would remain relatively constant. In its simplest form, Jacobs' tablet is a cylinder achieving equal inner surface by a multiplicity of cylindrical holes which are parallel to the axis of the outer cylinder, and accessed by the multiple apertures at each end of the cylinder. A related, but more sophisticated device, which now takes into account diffusion effects with respect to the inner surfaces, is that of Brooke, U.S. Pat. No. 3,851,648 (1974). Brooke discloses a cylindrical container, closed at the ends, with a cavity in the shape of a cylinder sector with the aperture in the form of a slot in the outer surface of cylinder (parallel to the axis of the cylinder), said slot at the apex of the cylinder sector cavity. See also Brooke et al., J. Pharm. Sci. 66, pp. 159-162 (1977). In practice, this device produces release rates which are initially high; Lipper et al., J. Pharm. Sci. 66, pp. 163-164 (1977). It is suggested that the device might be implanted into body cavities, but there is no suggestion for use of this device in the form of an ordinary tablet, or for a method of manufacturing such a tablet. Further, the device described by Brooke contains an inner compartment fully or partially filled with active substance leading to the surface of the device through a cavity.